1. Field of the Invention
This invention relates to diffusion coatings for components exposed to oxidizing and corrosive environments, such as the hostile environment of a gas turbine engine. More particularly, this invention is directed to a process for rejuvenating a diffusion aluminide coating without entirely removing the coating from a substrate.
2. Description of Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, though without a protective coating components formed from superalloys typically cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. One such type of coating is referred to as an environmental coating, i.e., a coating that is resistant to oxidation and hot corrosion. Environmental coatings that have found wide use include diffusion aluminide coatings formed by diffusion processes, such as a pack cementation and vapor phase processes.
Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition to form two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAI, where M is iron, nickel or cobalt, depending on the substrate material. The MAI intermetallic is the result of deposited aluminum and an outward diffusion of iron, nickel and/or cobalt from the substrate. During high temperature exposure in air, the MAI intermetallic forms a protective aluminum oxide (alumina) scale that inhibits oxidation of the diffusion coating and the underlying substrate. The chemistry of the additive layer can be modified by the presence in the aluminum-containing composition of additional elements, such as platinum, chromium, silicon, rhodium, hafnium, yttrium and zirconium. Diffusion aluminide coatings containing platinum, referred to as platinum aluminide coatings, are particularly widely used on gas turbine engine components. Platinum is typically incorporated into the coating by electroplating a layer of platinum on the substrate prior to aluminizing, yielding an additive layer that includes (Pt)NiAl-type intermetallic phases, usually PtAl2 or platinum in solution.
The second zone of a diffusion aluminide coating is formed in the surface region of the component beneath the additive layer. The diffusion zone contains various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. The intermetallics within the diffusion zone are the products of all alloying elements of the substrate and diffusion coating.
Though significant advances have been made with environmental coating materials and processes for forming such coatings, there is the inevitable requirement to repair these coatings under certain circumstances. For example, removal may be necessitated by erosion or thermal degradation of the diffusion coating, refurbishment of the component on which the coating is formed, or an in-process repair of the diffusion coating or a thermal barrier coating (if present) adhered to the component by the diffusion coating. The current state-of-the-art repair process is to completely remove a diffusion aluminide coating by treatment with an acidic solution capable of interacting with and removing both the additive and diffusion layers. An example of such a process is disclosed in commonly-assigned U.S. Pat. No. 3,833,414 to Grisik et al. The Grisik process relies on lengthy exposures to an aqueous solution of nitric and phosphoric acids, followed by treatment with an alkaline permanganate solution to completely remove the coating.
Removal of the entire aluminide coating, which includes the diffusion zone, results in the removal of a portion of the substrate surface. For gas turbine engine blade and vane airfoils, removing the diffusion zone can cause alloy depletion of the substrate surface and, for air-cooled components, excessively thinned walls and drastically altered airflow characteristics to the extent that the component must be scrapped. Therefore, rejuvenation processes have been developed for situations in which a diffusion aluminide coating must be refurbished in its entirety, but removal of the coating is not desired or allowed because of the effect on component life. Rejuvenation processes generally entail cleaning the surface of a component, followed by a controlled-activity aluminizing process that deposits additional aluminum on the component.
On occasion, excessive coating is deposited by rejuvenation processes, for example, the additive layer has a thickness in excess of about 100 micrometers. If the component has not been previously refurbished by completely removing its aluminide coating, the entire coating (i.e., additive layer and diffusion zone) can be fully stripped and the component re-aluminized. However, if the component has been previously refurbished by having its aluminide coating completely removed, thereby reducing its wall thickness, it may be necessary to scrap the component.
From the above, it can be appreciated that improved methods for refurbishing a diffusion aluminide coating are desired, particularly for those components that have undergone rejuvenation to have an excessively thick aluminide coating.